Bacterial translocation from the gut into the portal circulation can cause life-threatening infections in end-stage cirrhosis. Recent evidence suggests, however, that the liver and blood are already exposed to gut-derived microbial products at earlier disease stages. The relative contributions of microbe- and virus-induced intrahepatic immune cell activation to inflammation and severity of chronic viral hepatitis are not known. To examine the regulation of intrahepatic immune responses via the gut-liver axis, we are studying ex vivo and in vitro effector functions of innate immune cells (NK, NKT, MAIT cells and monocytes) in all three compartments of the gut-liver axis: systemic blood, portal blood and liver. Specifically, we are studying ex vivo and in vitro effector functions of these immune cells prior to, during (week 4 of therapy) and after HCV clearance (week 24 after stopping therapy). We are also studying portal vein blood obtained pre-treatment and at week 24 after stopping therapy. Results will be correlated with changes in liver histology and portal vein pressure and provide insight into the reversibility of liver disease. To perform mechanistic studies on the effect of the gut microbiome on host physiology and immune responses, we are using mouse models. We have reconstituted mice with different gut microbiota and investigated their effect on host immune responses in the context of diseases. The identification of beneficial host-microbe interactions and the characterization of their immunomodulatory mechanisms will increase our understanding on the regulation of intrahepatic and systemic immune responses in health and disease.